This chapter highlights the role of capnography as a monitoring tool with the different adjuncts to mechanical ventilation that are currently used in critically ill patients. The application of positive end-expiratory pressure (PEEP) is used to increase lung volume and improve oxygenation in patients with acute lung injury (ALI). Studies of unilateral lung injury demonstrate that the consolidated lung regions do not expand to total lung capacity during inflation. Tracheal gas insufflation (TGI) is an adjunct to mechanical ventilation that allows ventilation with small tidal volumes while CO2 is satisfactorily eliminated. High-frequency ventilation (HFV) techniques have three essential elements in common: a high-pressure flow generator, a valve for flow interruption, and a circuit for connection to the patient. Measurement of deadspace fraction early in the course of acute respiratory failure may provide clinicians important physiologic and prognostic information.